1. Field of the Invention
The present invention relates generally to image-forming apparatuses that develop an electrostatic image with a two-component developing agent containing a toner and a carrier. In particular, the present invention relates to an image-forming apparatus that forms an image using dark-colored toners and light-colored toners.
2. Description of the Related Art
A color image-forming apparatus that forms a multicolor image, such as a full color image, with toners of different colors has been widely used as an electrophotographic image-forming apparatus.
The requirements for image-forming apparatuses have become more stringent with advancement of the technology. There have been proposed image-forming apparatuses that use more colors than conventional four colors. For example, a technique widely employed in inkjet printing uses light-colored toners, such as a light cyan toner and a light magenta toner, in addition to common cyan, magenta, yellow, and black toners (see Japanese Patent Laid-Open No. 5-35038). Furthermore, there is a technique that uses a transparent toner in addition to the toners of four colors (see Japanese Patent Laid-Open No. 8-220821).
The main objective for adding light-colored toners is to form higher quality pictures by reducing dotted texture. The principle of forming an image using dark-colored toners and light-colored toners is described below by taking a cyan toner as an example.
FIG. 6 is a graph showing covering power of a light cyan toner (referred to as “LC toner” hereinafter”) indicated by a broken line and covering power of a dark cyan toner (referred to as “DC toner” hereinafter) indicated by a solid line. For example, the optical density of the LC toner is 0.7 when the amount of the toner loaded on a transfer material is 0.5 mg/cm2, and the optical density of the DC toner is 1.4 when the amount of the toner loaded on a transfer material is 0.5 mg/cm2. Based on a lookup table (LUT) for the LC toner shown in FIG. 7A and a lookup table (LUT) for the DC toner shown in FIG. 7B, an image is formed, (electrostatic image (latent image) based on the LUT is developed) using the LC toner and the DC toner. Note that the abscissa in each of FIGS. 7A and 7B indicates tone levels (levels 0 to 255) of the image before the image is divided into plates for the DC toner (dark-colored toner) and the LC toner (light-colored toner) (plate division). The ordinate in each graph shows the tone level (levels 0 to 255) after the plate division.
The phrase “image is divided into plates (plate division)” means to divide a set of image data of a particular color (also referred to as plate or channel) into two sets of image data for a dark-colored toner and a light-colored toner, respectively.
As shown in FIG. 8, the tone of cyan faithful to an image signal can be reproduced by superimposing the LC toner image and the DC toner image.
Although the principle of image formation has been described above using a dark cyan toner and a light cyan toner, substantially the same technique is employed in forming an image with a dark-colored toner and a light-colored toner of a color phase other than cyan.
Unlike image formation with a single dark-colored toner, the density per unit dot can be decreased and the dotted texture can be reduced by using a light-colored toner particularly for a low to intermediate-density portion of the image signal.
In general, a two-component development method in which a two-component developing agent mainly containing nonmagnetic toner particles (toner) and magnetic carrier particles (carriers) and a one-component development method in which no carrier is used are known as development methods employed in electrophotography. Image-forming apparatuses that use light-colored toners have an emphasis on image quality and thus frequently employ a two-component development method from the standpoints of high resolution and stabilizing the amount of loaded toner.
In a two-component development method, since the carrier comes into contact with an image-bearing member, a phenomenon called “carrier adhesion”, i.e., adherence of the carrier which is supposed to stay inside a developer onto the image-bearing member, sometimes occurs. There are two types of carrier adhesion: carrier adhesion outside the image region and carrier adhesion inside the image region. In particular, carrier adhesion inside the image region causes image failures such as voids caused by difficulty of transferring the toner around the carrier in a transfer unit and micro fixing defects (micro nonuniformity in gloss levels) caused by difficulty of fixing the toner around the carrier in a fixing unit. Accordingly, carrier adhesion inside the image region is more problematic than carrier adhesion outside the image region.
For example, Japanese Patent Laid-Open Nos. 61-160764 and 1-92759 each disclose a technique of overcoming the problem of carrier adhesion by increasing the magnetic binding force between the carrier and a developing magnetic pole of a magnet fixed inside a developing agent-bearing member in a developer.
However, when the magnetic binding force between the developing magnetic pole and the carrier increases, the magnetic binding force between carrier particles on the developing agent-bearing member also increases, which increases the strength of the carrier chains. As a result, the image on the image-bearing member is easily disturbed as the carrier chains contact the image, and there is a risk of image failures such as roughness (dotted texture) in the low-density portion.
Due to this reason, existing color image-forming apparatuses mainly using yellow, magenta, cyan, and black use a magnetic binding force within an extent that does not cause image failures such as roughness while minimizing carrier adhesion.
It should be noted here that Japanese Patent Laid-Open No. 2003-280460 teaches a structure including a two-component developing unit containing a coloring agent and another two-component developing unit not containing a coloring agent per electrostatic latent image-bearing member. For one electrostatic image, the region outside the image region is developed by the developing unit not containing a coloring agent and the image region is developed by the developing unit containing a coloring agent. Adhesion of the carrier outside the image region is avoided by making the particle diameter of the carrier of the developing agent not containing a coloring agent larger than the particle diameter of the carrier of the developing agent containing the coloring agent.
However, an image-forming apparatus that uses a light-colored toner and a dark-colored toner has suffered from the following problem. When the same type of carrier is used in developing agents of all colors, the frequency of carrier adhesion in the image region is sometimes significantly higher in a developer using a light-colored toner than in a developer using a dark-colored toner. The reason therefor is as follows.
The mechanism of carrier adhesion onto the image region will now be described with reference to FIGS. 9 and 10. FIG. 9 is a schematic diagram of a developing unit (developing region) of a developer. FIG. 10 shows latent image potentials during formation of a solid image (image with the maximum density level) by reversal printing.
FIG. 9 schematically shows a photosensitive drum 1 that functions as an image-bearing member, a developing sleeve 41 that functions as a developing agent-bearing member, a magnet 42 that functions as means for generating a magnetic field, toner particles, and carrier particles. The magnet 42 has a developing magnetic pole S1 in a developing region D between the photosensitive drum 1 and the developing sleeve 41 opposing each other. Herein, the case in which an electrostatic image formed on a negatively charged photosensitive drum 1 is reversely developed with a negatively chargeable toner is described as an example.
During formation of a solid image, a latent image potential shown in FIG. 10 is formed on the photosensitive drum 1. A negatively charged toner is supplied onto the photosensitive drum 1 due to the potential difference Vcont between the potential (Vdc) of the developing sleeve 41 and the potential of the exposed region (VL).
Meanwhile, inside the carrier chain exposed to the potential difference, negative charges are injected at ends of the carrier chains by the voltage difference Vcont. If negative potentials are accumulated in the carrier in a predetermined amount or more, the negatively charged carrier particles will adhere on the photosensitive drum 1 due to the force of the electric field in the same manner as in a typical developing process using a toner.
Thus, carrier adhesion onto the image region tends to frequently occur during the development of a high-density toner image with a large potential difference Vcont. As a consequence, the frequency of carrier adhesion onto the image region during the development with a light-colored toner becomes higher than that during the development with a dark-colored toner, as described below.
That is, as previously described, image formation using a light-colored toner and a dark-colored toner is conducted based on the lookup tables (LUTs) shown in FIGS. 7A and 7B. For a typical average operational density (e.g., an average density of 100 to 140 in an image signal levels of 0 to 255), the level of the image output signal for the light-colored toner is 200 to 255, which is equivalent to high-density development for forming a solid image. In other words, the potential difference Vcont in this case is relatively large.
In contrast, the level of the image output signal of the dark-colored toner for the typical average operation density described above is equal to the level for low-density development. In other words, the potential difference Vcont in this case is relatively small.
Therefore, in forming an image with an average density, the possibility of carrier adhesion on the image region during the development with a light-colored toner is several times greater than that during the development with a dark-colored toner.
It should be noted that the amount of the toner loaded to form a light-colored toner image having an average density can be decreased by decreasing the output level of an intermediate portion of the image signal in the lookup table for the light-colored toner. In this manner, it is possible to decrease the frequency of toner adhesion on the image region during development with the light-colored toner. However, in such a case, the original advantage of using the light-colored toner, i.e., to decrease the dotted texture in the low-density portion, is impaired.